Electric cooking appliances such as ovens, for example, have traditionally included a heating element that, when supplied with electric energy generated a sufficient amount of heat to cook food. Due to the high power demands of the heating element, isolation devices have typically been employed between sensitive control circuitry and the high-power components associated with the heating element. The isolation devices insulate the control circuit components from the high power being delivered to the heating element.
One such isolation device commonly found on electronic cooking appliances is a relay. A relay is simply an electrical switch that uses an electromagnet to selectively toggle the high-power supply on and off to respectively activate and deactivate the heating element while cooking. Electric current flows through a coil to generate a magnetic field that, in turn, pulls a magnetic switching element such as a metallic armature into contact with one or more terminals of the high-power circuit including the heating element. This contact between the switching element and the terminal(s) of the high-power circuit closes the high-power circuit (for a normally-open relay), thereby supplying the high-voltage and/or high-current electric energy to the heating element. Inducing a magnetic field in such a manner allows the relay to close the high-power circuit without establishing a conductive pathway to the more-sensitive control circuit that controls the operational state of the relay, thereby isolating the two circuits.
Although the relay provides sufficient isolation to protect the control circuit from the high power of the circuit including the heating element, the relay has its own shortcomings that can limit its application in consumer goods such as cooking appliances. In a cooking appliance, the relay will be cycled on and off thousands of times, if not more, during its lifetime to control the delivery of the high AC voltages required to energize the heating element and maintain a user-selected cooking temperature. If the relay opens the circuit delivering the high AC voltage to the heating element while the voltage waveform is at or near its peak value, then a considerable arc will be generated between the relay contacts when the high-power circuit is opened. Arcing damages the relay contacts and can eventually shorten the relay's useful life if repeatedly subjected to such arcs.
In order to minimize arcing experienced between the relay contacts, efforts have been made to time the opening of the relay to correspond to the zero-crossing of the AC voltage waveform. However, the relay coil is also an inductor that stores electric energy and resists rapid changes in the current flowing through the relay coil with respect to time. The resistance to instantaneous changes in current flowing through the relay coil makes the relay slow to open, which can extend the length of time that an arc extends between the relay's contacts. Some relays have been designed to open rapidly to minimize the time an arc exists between its contacts, but such relays are expensive, making them impractical for use in cooking appliances. Other relays, although cheaper, take many (often more than 10) milliseconds to open. But since each half cycle of a 60 Hz AC voltage waveform lasts about 8.3 milliseconds, each time the high-power circuit is opened, the arc will experience at least one peak voltage across the relay's contacts, and possibly more. An arc between the contacts at the peak voltage of the AC waveform imparts the most damage on the contacts, and significantly shortens the useful life of the relay.
Accordingly, there is a need in the art for a cooking appliance with improved electrical isolation between control and high-power circuits to minimize damage to the electrical isolation device while transitioning between on and off states.